One of the exciting things about doing research is the interesting types of tools available for use (like lasers – Pew Pew!!!). Many people are familiar, or at least have heard about or seen, wonderful images produced electron microscopes – tools that allow us to see extremely small things.  What I would like to talk about here is the equally cool but less well-known types of tools and techniques that allow us to observe not just extremely small things, but observe things that happen ridiculously fast. Things that are in all seriousness called Ultrafast by the scientific community.

How Fast is Ultrafast?

Well it’s pretty darned fast. Ultrafast typically refers to looking at molecular processes that happen on the timescale of femtoseconds. Ok, I realize femtoseconds may sound pretty fast but most people don’t have a conception for how fast that actually is. I’ll break it down.

  • There are a thousand milliseconds in one second
  • and a thousand microsceonds in one millisecond
  • and a thousand nanoseconds in one microsecond
  • and a thousand picoseconds in one nanosecond
  • and finally a thousand femtoseconds in one picosecond

Phew, so that means there are 1,000,000,000,000,000 femtoseconds in a second.

It also means that light – which travels at a speed of roughly 300,000,000 meters per second – takes 3,333 fs to travel a distance of one millimeter.

To put it one final way, there are as many femtosecond in one second as there are  seconds in 31 million years.

So yes – quite fast!

Why Bother?

It is reasonable to ask why one would want to look at things on a femtosecond timescale. Wouldn’t everything appear to be at a stand-still? While this would be true for looking at macroscale objects that we interact with everyday, things are much different on the molecular level. Certain processes – like the movement of electrons from one place to another after absorption of light, or certain high frequency vibrations within the molecule – only occur on the femto or picosceond timescale. Say for instance you are designing a dye molecule that absorbs light and can be used in a type of solar cell. You’d like to know what happens to the electron right after light is absorbed. Does instantly go back to the ground state? Does it go to a different part of the molecule where it could be extracted to do useful work? If we wish to understand these mechanisms, we have to have a tool that can record events on the same timescale that they occur.

Pew pew! Look at me working on the laser! Unfortunately it doesn't go pew pew at all...

How to Measure: Use the Proper Ruler

To measure things at this speed you need the appropriate tool. As an example, say you want to take a picture of your favorite NASCAR driver as he speeds around the racetrack. You snap the photo with your cheap digital camera and when you look at the result all you can see is a long streaking blur.

The photo of the car is blurry because your camera took a photograph on a timescale that was slower than the motion of the car.

This is because the camera is slower than the motion of the car. When looking at the picture, you’re not exactly sure where the car was when the picture was taken. You know it’s somewhere within the blur, but not to any precision. We would say that your photograph has poor time resolution.

Now your friend is megarich and has a top of the line digital-SLR-extreme camera which can take pictures much more rapidly. You take a look at her photo and you see a nice crisp and clean image of the car. This is because the camera took a picture faster than the motion of the car, so the photo shows exactly the position of the car at the time that the picture was taken.

Here the photograph of the car is crisp, and you can tell exactly where the car is because the camera took a picture faster than the motion of the car.

Observing things on the femtosecond timescale requires that you have a tool that can also operate at that speed. As mentioned certain processes in molecules, such as electron hopping from one part of the molecule to another, or a high frequency vibration of a chemical bond, can occur on femtosecond timescales. In order to observe these motions we require a tool that can also record information as fast as it is occuring. Unfortunately the best electronics can only operate on a nanosecond timescale, or one million times slower than the femtosecond timescale needed. So instead we create and use laser pulses that are only femtoseconds long in order to observe the interesting ultrafast phenomena that occur in molecules.

Too Long; Didn’t Read:

Ultrafast phenomena occur on a timescale of femtoseconds. Observing these processes requires a tool that can operate that fast. Electronics can’t keep up so we use ultrafast laser pulses! Pew Pew!

Next time I’ll  explain how to produce ultrafast laser pulses and present some ultrafast data I’ve collected. Hope you enjoyed reading, leave any questions or comments below!


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